Attention has recently come to be attracted to special carbon fibers called nanocarbon fibers. Nanocarbon fibers are substances shaped like cylindrically wound sheets of carbon atoms arranged in a hexagonal mesh and having a diameter of 1.0 to 150 nm (nanometers) and a length of several to 100 μm. These substances are called, e.g., nanocarbon fibers or nanocarbon tubes (hereinafter referred to as nanocarbon materials), since they have a nano-sized diameter.
The nanocarbon materials comprise a material of high thermal conductivity, as well as a reinforcing material, and can improve the thermal conductivity of a metallic material in which it is mixed.
The nanocarbon materials provide an improved thermal conductivity when they extend in the direction in which heat is conducted. Thus, a method in which nanocarbon materials are arranged in a certain direction has been proposed by JP-A-2004-131758.
The proposed method will now be described with reference to FIG. 5. FIG. 5 shows a cooling drum 101, a groove 102 formed around the cooling drum 101, a container 103, a molten material 104, a solidified material 105, a rolling mill 106 and a cutter 107.
The molten material 104 prepared by mixing nanocarbon materials in molten aluminum is fed from its container 103 to the groove 102 on the cooling drum 101 at a constant flow rate. The cooling drum 101 is rotated at a high speed giving it an outer peripheral velocity which is higher than the flow rate of the molten material 104.
The molten material 104 is, therefore, drawn along the groove 102 and the nanocarbon materials are oriented in the direction in which the molten material is drawn. At the same time, it is cooled and solidified into the solidified material 105.
The solidified material 105 is rolled by the rolling mill 106 and cut by the cutter 107 to give rod-shaped materials 108. The rod-shaped materials 108 have a thickness of 0.1 to 2.0 mm. The rod-shaped materials 108 have their thermal conductivity elevated drastically along their length by the nanocarbon materials oriented along their length.
However, a large amount of heat energy is consumed to heat aluminum to its melting point to prepare the molten material 104.
If the cooling drum 101 is rotated too fast, the molten material 104 is torn and if it is rotated too slowly, the nanocarbon materials fail to be oriented uniformly. Thus, the rotating speed of the cooling drum 101 requires difficult control. The solidification of the molten material 104 cooled on the cooling drum 101 proceeds from its surface to its center. When a material containing a foreign substance solidifies from its surface to its center, the foreign substance (nanocarbon materials in the context of the present invention) tends to gather in the center. Thus, the nanocarbon materials lack uniformity in distribution and give a composite product of lower strength. The deficiency of nanocarbon materials in the skin of the product lowers its surface hardness and wear resistance.
Accordingly, the known method in which the molten material 104 is drawn by the cooling drum 101 needs to be improved in the control of the rotating speed of the cooling drum and the surface hardness of the product.